Articles of footwear usually include an upper and a sole assembly. The upper can include sections of thin material, straps, laces, and the like for covering the wearer's foot and securing the footwear to the wearer. The sole assembly can include an outsole that is typically a unitary piece of relatively durable, high-friction material that provides traction for the footwear. Also, the sole assembly can include a midsole including foam, fluid filled bladder(s), etc. for providing cushioned support for the wearer.
The sole assembly can resiliently deform in response to loads from the wearer. For instance, walking or running can cause the sole assembly to resiliently flex and bend to thereby maintain sufficient surface contact with the ground. The sole assembly can also resiliently deflect in a direction perpendicular to the ground in order to absorb weight loads, thereby cushioning the wearer and providing greater comfort.
In some cases, the sole assembly can include deep grooves (e.g., “flex grooves”) or sipes that separate the sole assembly into discrete sole elements and that increase the flexibility of the sole assembly for improved performance. More specifically, the midsole can include one or more grooves with substantially V-shaped cross sections and the depth of the groove can extend through the majority and/or all of the thickness of the midsole. (See e.g., U.S. Pat. No. 4,562,651, issued Jan. 7, 1986 to Frederick, et al., U.S. Pat. No. 6,055,746, issued May 2, 2000 to Lyden et al., U.S. Pat. No. 6,990,755, issued Jan. 31, 2006 to Hatfield at al., U.S. Pat. No. 7,171,767, issued Feb. 6, 2007 to Hatfield et al U.S. Pat. No. 7,290,357, issued Nov. 6, 2007 to McDonald et al., U.S. Pat. No. 7,392,605, issued Jul. 1, 2008 to Hatfield at al., and U.S. Pat. No. 7,607,241, issued Oct. 27, 2009 to McDonald at al., each of which is incorporated herein by reference.) As such, the sole assembly can readily flex and bend about the flex groove, allowing the sole assembly to better conform to the wearer's foot, facilitate flexing during movement of the foot, and the like.
Although conventional sole assemblies have been adequate for their intended purposes, they do suffer from certain disadvantages. For instance, the resiliency and/or flexibility of the midsole may not be suitable for a wide range of activities. More specifically, the midsole may be relatively stiff, and as such, the footwear may be suitable for walking, however, this same midsole might be too stiff for playing tennis, running, or other activities. Accordingly, the usefulness of the footwear may be somewhat limited. On the other hand, the midsole may be highly flexible to be useful for running, playing tennis, and the like; however, this same midsole might be too flexible for other activities in which a stiffer sole provides greater comfort.
Additionally, the sole assembly can wear over time. As such, the midsole may lose resiliency, and the sole assembly may grow more flexible over time, which can be unwanted and undesirable. As a result, the useful life of the footwear may be prematurely shortened. Also, the midsole may develop a curvature over time such that the toe area of the footwear curves upward away from the ground, and the footwear may be less aesthetically pleasing as a result.